The ground-state average and equilibrium structures of formaldehyde and ethylene

Abstract

The use of recently determined highly precise rotational constants for various isotopes of formaldehyde and ethylene enables the ground-state average structures to be determined to high accuracy. Harmonic corrections to the inertial constants are calculated using recently determined general harmonic force fields. In the case of formaldehyde, the data enable changes in all three structural parameters due to isotopic mass effects to be determined. For ethylene, only the change in CH bond length on deuteration is well characterized, due to the somewhat lower accuracy in the rotational constants. Using isotopic structural differences transferred from formaldehyde, and supported by calculations, ground-state rotational constants for all deutero isotopes of ethylene are predicted to what is considered to be better than 0·1 per cent reliability.

The change in CH bond length on deuteration in ethylene is less than in formaldehyde. This is supported by calculations, although it appears also that the CH bond stretching coordinate in formaldehyde is more anharmonic than that in ethylene. The equilibrium structures are predicted in terms of the observed δr _z(XH-XD) structural parameters. For formaldehyde, the structure accurately reproduces the experimental equilibrium inertial constants of H₂CO. For ethylene the structure is in good agreement with that determined by making the corresponding corrections to the independently determined electron diffraction r_g structural parameters.